1. Field of the Invention
The present invention relates to a device and method for multiplication of repetition frequency in optical pulse trains. Especially the present invention provides the optical pulse generating device and method which multiplies repetition frequencies of the optical pulse trains generated within ML-FRL with high stability.
This invention is a scheme that can multiply pulse repetition frequency in active harmonically mode-locked fibre ring lasers and therefore making it as an optical pulse source for generating optical pulse trains at a repetition rates which is much higher than the applied modulation signal frequency. The high repetition optical pulse trains are important for the high-speed optical communication systems based on optical time-division multiplexing (OTDM) technology etc. Further, the present invention generates stable electrical signals at microwave frequency band, millimeter-wave frequency band, and higher frequency bands (>100 GHz).
2. Description of the Related Art
Short optical pulses at high repetition rates are important for future ultra high-speed optical communication systems (see references 1, 2). Active harmonically mode-locked fibre ring lasers (ML-FRL) have become very popular optical pulse source for OTDM/WDM based high-speed optical communication systems due to their ability to generate short and transform-limited optical pulses at high repetition rates. Here an RF modulation signal which is an harmonic of the cavity resonance frequency is applied to the RF port of the Mach-Zehnder intensity modulator (MZM) placed inside the cavity and biased at the quadrature point on its transmission characteristic curve, thereby generating optical pulses at a repetition frequency equal to the applied RF modulation signal frequency. However in such lasers, maximum pulse repetition frequency is limited by the modulator bandwidth as well as frequency of the drive electronics. In order to simplify the architecture of such high-speed communication systems, it is important that the base pulse rate is increased further. Several methods have been proposed to increase the pulse repetition frequency in ML-FRL while relaxing the requirement of large bandwidth MZM and RF drive electronics. These include frequency multiplication in intracavity Mach-Zehnder intensity modulator (MZM) where nonlinear characteristic of the modulator was exploited to increase pulse repetition frequency (see references 3, 4), and intracavity optical filtering via fibre Fabry-Perot (FFP) filter where the concept of selective filtering of the oscillating longitudinal modes was successfully utilised to increase pulse repetition frequency in ML-FRL (see references 5, 6). In another method, modulation frequency was rationally detuned by ±fc/p to generate optical pulse trains at a repetition rate of p×fm in ML-FRL (see references 7-9).
FIG. 11(a) shows a prior art of the ML-FRL. In FIG. 11(a), an optical amplifier A comprises an excitation light source and a gain medium having optical amplifying characteristics, the gain medium 1 is comprised of Er/Yb doped fibres. A modulator 4 is composed of Mach-Zehnder modulator. An optical coupler 9 divides the light generated in the optical fibre ring to an optical sensor 23 by a rate of 10 to 90 (10% of whole generated light is passed to the optical sensor). An electric oscillator 21 generates high frequency electric signals. A measuring instrument 24 measures electric signals converted from the optical signals.
In the system of FIG. 11(a), the laser beam generated with the excitation light source 2 excites the Er/Yb doped fibres (EDF) that is a gain medium. As a result, optical signal oscillates in the fibre ring cavity at frequency which is a resonant frequency of the fibre ring cavity and the integer multiple of the resonant frequency, that is super harmonic mode. The electric signal generator 21 generates electric signals of frequency fm, which is integer multiple of the cavity resonant frequency fc, and the frequency fm is applied to the modulator that is Mach-Zehnder optical intesity modulator 4. The modulator 4 is biased at voltage of Vb.
In the system of FIG. 11A, the repetition frequency of the optical pulse train generated in the fibre ring is equal to the applied modulation frequency fm when the modulator 4 is biased at its quadrature point on its transfer characteristics curve. FIG. 11B shows the pulse train generated by the above-mentioned system.
The concept of composite cavity structure was first proposed as a means to suppress supermode noise in ML-FRLs (see reference 10). As discussed earlier, one of the method of increasing pulse repetition frequency in ML-FRL is by using intracavity optical filtering in ML-FRL where a FFP filter with a free spectral range (FSR) equal to K×fm was inserted in the ML-FRL cavity, where K is an integer, and fm is the applied modulation frequency. As a result, under cw operating condition, the laser cavity supports dominant cavity resonance modes which are frequency spaced at FSR. This in turn will lead to the generation of optical pulse trains at repetition rates equal to the FSR under harmonic mode-locking (see references 5, 6). In this method the maximum pulse repetition frequency is limited to the FSR of the intracavity FFP filter.
As mentioned above, the repetition frequency of the optical pulse train in the ML-FRL is limited to the integer multiple frequency of the frequency fm, that is K×fm(K: integer).